Field of the Invention
The present invention relates to a wireless communication system for receiving data transmitted from a plurality of transmitting apparatuses through wireless communication means.
Description of the Related Art
In a known surveillance camera system, each of a plurality of surveillance cameras takes an image of a monitored object, and the captured images are output to a monitor at the same time to display the captured images. In the conventional surveillance camera system, video signals obtained by image capture by use of the surveillance cameras are compressed in, for example, H.264 format and are then divided into RTP (Real time Transport Protocol) packets. The RTP packets are transmitted to a receiving apparatus through a wireless communication network, and the receiving apparatus transmits the RTP packets to a plurality of decoders to decode the RTP packets. Decoded captured images are combined and displayed on the monitor, thereby it is possible to display the monitored images of plural points at the same time.
The communication environment in the wireless communication network always changes because of some factors such as obstacles and weather change. Therefore, RTP packets from a plurality of surveillance cameras may not be transmitted to the receiving apparatus at the same time, and for example, the transfer of the RTP packets from a surveillance camera may be delayed. In the event of such transmission delay, monitored images displayed on the monitor are deviated. For example, the monitored images from a plurality of surveillance cameras to capture images at different points may display the same object.
In a video monitoring system described in Japanese Patent Laid-Open No. 2011-29969, an NTP (Network Time Protocol) or the like is used to hold common time information between a plurality of surveillance cameras. The common time information is added to image data obtained by imaging, and then the image data is transmitted. A receiving apparatus synchronizes the transmitted image data based on the added time information to prevent a deviation in the display time.
According to the technique described in the above Japanese Patent Laid-Open No. 2011-29969, a time synchronization process between the surveillance cameras is necessary before the system starts the operation, and the time synchronization process is necessary again when the time information between these surveillance cameras is deviated. This complicates the system operation. In the technique described in Japanese Patent Laid-Open No. 2011-29969, in order to realize simultaneous display of the early-transmitted image and late-transmitted image, the decoder needs to include a buffer that holds early-transmitted image data, which may cause increase in the system cost.
The present invention has been made in view of the foregoing circumstances, and an object of the present invention is to provide a wireless communication system that reproduces and displays video data transmitted from a plurality of wireless terminals, wherein transmission of a plurality of video data can be synchronized based on a simple configuration.